1. Field of the Invention
The present invention relates to plant fertilization systems, and more particularly to the use of microorganisms for extracting phosphorus from soil, rock phosphate, or commercially refined phosphate sources and distributing these materials to plant root systems without absorptive loss to clay minerals and other soil constituents. The present invention also relates to the use of microorganisms in plant nourishment and, more particularly, to an improved method and apparatus for producing, packaging and deploying beneficial symbiotic mycorrhizal fungi in association with crop or other plants to enhance recovery of phosphorus, water and other constituents from the soil.
2. The Prior Art
It is well known that phosphorus is one of the elemental constituents essential to the growth of plants. It is also generally known that phosphorus must be in a water soluble form in order to be utilized by plants and that most soils are deficient in such soluble forms of phosphorus.
Nevertheless, many soils have moderate to high levels of insoluble or unavailable phosphorus in the form of natural minerals, organic phosphorus compounds, and clay minerals containing adsorbed phosphorus.
In order to satisfy the continuing agricultural need for soluble phosphorus, commercial refinement processes have been utilized to obtain soluble phosphorus from rock phosphates which are soluble in acid. Such processes are not only complex and energy-intensive, but they require "high-grade" rock phosphate, the natural United States sources of which are rapidly becoming depleted.
A high proportion, often in excess of sixty percent (60%), of the soluble phosphorus applied to argillaceous soils is lost due to the rapid adsorption of phosphorus by clay minerals and iron, or by its incorporation into organic compounds and other soil constituents. The phosphorus becomes irreversibly adsorbed by the clay minerals, leading to permanent loss of much of the soluble phosphorus applied to the soil.
A further problem with direct application of phosphorus to the rhizosphere of the plant root system is a condition referred to as phosphorus toxicity. This condition occurs when the phosphorus within a plant exceeds optimal levels and may result in the stunting of the plant's growth. Thus, the application of an initially high level of phosphorus to the soil may, in fact, cause decreased growth of plants.
In seeking to overcome these problems, attempts have previously been made to utilize unrefined rock-phosphates or microorganisms or combinations thereof in a soil environment near plant root systems to provide suitable phosphorus to those root systems. These attempts have been based on the understanding that soluble phosphorus is slowly made available to plants by the breakdown of natural minerals and organic phosphorus compound through the action of microorganisms, as well as by organic and inorganic acids. Such attempts are reflected in the disclosures of U.S. Pat. Nos. 947,795, 1,361,597, and 4,155,737. However, these attempts have not succeeded in creating a microenvironment suitably capable of growing phospholytic microorganisms or breaking down certain phosphorous compounds by naturally occurring acids.
Some of the prior attempts to overcome the abovedescribed problems have been able to provide a source of soluble phosphorus, which may be placed in the soil, however, no means has been provided for preventing loss of this soluble phosphorus through adsorption by clay minerals.
In the past, attempts to produce soluble phosphate from existing insoluble sources within the soil have generally been unworkable. Most soils in temperate climates do not provide the conditions of low pH which are conducive to the spontaneous and direct conversion of insoluble forms of phosphorus to soluble forms of phosphorus. Insolubility in most of these instances is related to a neutral or alkaline soil pH. Thus, the problem of providing a substantial mineral or other natural source of soluble phosphate for the plant root system has not been suitably solved by the prior art.
It is well known that plants grown in soils which are deficient in soluble phosphorus benefit from association with mycorrhizal fungi (hereinafter referred to as MF). Such associations are formed by fungi belonging to the family Endogonaceae which constitute two morphological groups: (1) Ectomycorrhizae form principally on the roots of woody plants which include economically important families of forest trees. (2) Endomycorrhizae, also commonly known as vesicular-arbuscular mycorrhizae (hereinafter VAM), colonize the roots of most food crop plants and are the principal subject of this patent application. Both the endomycorrhizal and ectomycorrhizal fungi develop symbiotic associations with feeder roots of the plant root system, receiving carbohydrates and other nourishment from the plant while directly benefiting the plant by transferring to it phosphorus, water, and other constitutents from the soil through a hyphal network.
The efficiency of phosphorus uptake from soils deficient in soluble phosphorus is greater for MF than for root cells of the host plant. This is because the MF have a lower threshold for phosphorus retrieval. Furthermore, the ability of the fungus to form an extensive hyphal network in the soil surrounding the plant root permits phosphorus recovery from a large volume of soil since there may be up to 80 cm of hyphae for each cm of root infected by vesicular-arbuscular endomycorrhizal fungi. Individual hyphae project up to 10 cm from the root system of the host plant, thereby penetrating the zone of depletion adjacent to the root cortex and greatly extending the effective volume of soil from which phosphorus, water and other constituents can be extracted.
Phosphorus transfer from the soil to the plant is accomplished within MF by a process termed translocation. MF first concentrate the extracted phosphorus in polyphosphate granules within the cell cytoplasm and the move the granules within the hyphae of the external hyphal net by means of bulk flow and possibly by means of active transport using cytoskeletal elements. Soluble phosphorus is delivered into the plant through an internal hyphal net whose specialized arbuscles are intimately associated with cells of the root cortex. In vitro laboratory tests have demonstrated that these fungi may also be capable of translocating soluble phosphorus from a plant with high levels of phosphorus to a plant which is deficient in phosphorus; however, attempts to demonstrate this in the field have been inconclusive.
The main problem encountered in attempting to use endo-MF for transferring soluble phosphorus is that it has not been possible to produce these fungi in sufficient quantities to make their use feasible except on an experimental basis. It has not been possible to induce endo-MF to propogate and ensporulate by means of in vitro culture techniques; moreover, methods of producing large quantities of fungal concentrates have also not been developed. Further, storage for extended periods of time or transportation of active fungi has been impractical since active fungi must be retained either in culture or in soil. Since neither growth nor reproduction has been achieved by means of culture methods, storage of the endo-MF for extended times is impractical because of the extensive physical care required in keeping the endo-MF active in the in vitro culture medium or in soil. Incorporation of either moist or lyophyllized (freeze-dried) propagules of endo-MF into pellets formed of soil, bentonite clay or calcium carbonate have not provided as efficient mode of plant inoculation as pre-inoculation or the use of large quantities of active endo-MF situated beneath the root system of an uninfected plant. Further, it has been found that direct introduction of endo-MF into many soils is not possible because these fungi are not able to compete with existing soil organisms, and because suitable conditions for their establishment within the soil environment do not exist without association with a plant host.
Thus, none of the above-described methods and devices provide a full and workable solution to the problem of supplying soluble phosphorus to plant root systems either in sufficient quantity or without loss through adsorption by clay minerals or other constituents of the soil. What is needed to fully overcome the deficiencies of the prior art is a means for the controlled mobilization of soluble phosphorus from one or more sources and its transfer to plant root systems without appreciable loss to clay minerals and other adsorptive constitutents within the surrounding soil. It would be a further important improvement to provide a means for supplying phosphorus to plant systems from unrefined sources of rock phosphate. Another desirable improvement would be to provide a phosphorous supply which would be available over an extended period of time, such that the plant nutrient requirements are met without phosphorus toxicity. A still further advance in the art would be to provide a means for balancing the phosphorus levels so as to transfer phosphorus from plants which have excess phosphorus to those deficient in phosphorus.
In accomplishing this, what is also needed is a practical means by which VAM fungi can be used for encouraging plant growth in commercial applications. This would require a method for mass producing efficient species of MF in sufficient quantities for large-scale distribution. This would also require that the fungi remain viable, even after storage, transport and implantation into the soil. In addition, the fungi would also have to rapidly colonize the roots of host plants so that the latter may benefit from the transfer of phosphorus and other soil constituents.